As one of the main types of wind power generation, doubly-fed induction generator (DFIG) has been widely used with the rapid development of new energy. In this paper, the mechanism of insufficient stability of DFIG control system under weak grid is investigated. By constructing small signal model of DFIG in weak grid, the influence loop of phase-locked loop (PLL) on current closed-loop control is analyzed. Based on the small signal model, a compound decoupling control strategy considering the effect of PLL is proposed, which effectively improves the stability margin of DFIG in weak grid. The effectiveness of the proposed strategy is verified by experiments.

随着风力发电机的大规模并网发电,并网点的短路比不断降低.风电机组并网功率增大使得线路阻抗上的压降上升,并网点电压也随之降低,进而导致风力发电机的输出功率受到限制.为改善风机并网在低短路比下电压降低致使输出功率受限的问题,以双馈风力发电机为研究对象,提出了一种基于电压主动支撑的闭环优化控制策略.该方法在实现对并网点电压的支撑的基础上,提高了双馈风电机组在低短路比下的功率传输能力,同时解决了常规电压闭环控制中由功率耦合引发的系统稳定性问题.在优化控制策略下,系统的主导极点阻尼比更大,使得系统更加稳定.最后,在11 kW的双馈电机实验平台上验证了优化控制策略下双馈电机的阶跃响应稳定性更高.

区别于跟网型结构,采用构网型结构的双馈风电机组不存在电流环与锁相环的耦合,且具备一定的频率/电压主动支撑能力,因而更适合在弱电网下运行.针对双馈风电机组的构网型结构,提出一种磁链控制型双馈风电机组(FC-DFIG)构网方案,通过构建定子磁链外环-转子电流内环的双闭环结构实现对定子磁链矢量的幅相控制,进而调节并网功率.基于传输功率与定子磁链矢量的关系,提出FC-DFIG构网方案,并建立其小信号模型,证明该结构稳定可控.此外,针对弱电网下的低阻尼特性,基于定子d轴磁链微分前馈策略优化系统阻尼,理论和实验证明所提方案可以改善机组阻尼特性,有效提升FC-DFIG在弱电网下的稳定性.

This article proposes an improved virtual inertia control strategy, which reduces the lag effect of a low-pass filter in conventional control and increases the speed of virtual inertia control in wind turbines. The grid frequency obtained by phase locked loop (PLL) must pass through a low- pass filter with a small cutoff frequency under distorted grid voltage. The delay caused by the low- pass filter conflicts with the goal of fast support required for virtual inertia control. This article analyzes the influence of filter on system stability and response speed in frequency detection. First, the advantages and disadvantages of differential feed-forward (DFF) control are proposed and analyzed. Then, a lag-lead-based phase compensator (LLPC) scheme is proposed to provide better system stability and rapidity. Finally, the effectiveness of the proposed control strategy is validated on an 11-kW doubly fed induction generator (DFIG) platform.

With the rapid development of wind power, the voltage source DFIG (double-fed induction generator) has the ability to actively provide frequency and voltage support under weak grids and has attracted wide attention. However, when it is connected to the parallel-compensated weak power grid, medium and high frequency resonance may occur due to the interaction of the machine and the network. In response to this problem, this paper first establishes a generator-side impedance model of a voltage source DFIG, and analyzes the resonance phenomenon in the parallel compensation grid. Then, based on the control structure characteristics of the voltage source DFIG, a resonance suppression strategy is proposed. Finally, the resonance suppression strategy is verified by simulation.

Large scale integration of wind farm combined to the grid is a tendency for the development of the new energy, which causing the issue of low system inertia. Though analyzing the mode of operation and the Maximum Power Point Tracking strategy (MPPT) of double fed induction generator (DFIG), reviewing advantage and disadvantage of scholars' works on inertia control in the past few years, this paper proposed a new strategy to mode the DFIG, that is over speed and torque reserve. The wind farm changes its electromagnetic power depending on the frequency of the gird, as a result, the reduced the drop of frequency and increase the inertia of the system. On the other hand, the farm coordinates primary frequency regulation and avoids the second frequency drop. This paper builds a system with Simulink to validate the strategy, and the results confirmed its effectiveness.